1. Field of the Invention
The invention applies to image sensor devices formed by photosensitive semiconductors and more particularly to such devices comprising at least one "large-sized" photosensitive surface.
2. Description of the Related Art
The image sensor devices with semiconductors most often comprise either multiple photosensitive elements placed in lines and in columns, or a single line of these photosensitive elements which then constitute a strip. Each photosensitive element corresponds to an elementary image point, and the dimensions of this elementary image point are linked to those of the photosensitive surface of the sensor element. In the description below, the photosensitive surface of a sensor element is called "pixel." It should be noted that in some cases, the image sensor device can comprise a single photosensitive element, i.e., a single pixel.
The pixels have dimensions which vary with the application. For example, when an effort is made to obtain a high image resolution, pixels of small dimensions (for example, 10 microns.times.10 microns) are used. In other cases, pixels called "large sized" (for example, on the order of 100 microns.times.100 microns) are used, in particular in an application in communications between satellites.
The semiconductive photosensitive elements are now photodiodes or else elements produced according to MOS (from "Mental Oxide Semiconductor") technology. In MOS technology, a capacitance is produced by depositing, on a semiconductive substrate, an insulating layer formed by an oxide, and covered by a conductive layer. This conductive layer constitutes an electrode and it is often made of a polycrystalline silicon layer. According to this MOS technology, a sequence of such capacitances able to form a sequence of stages of a shift register of the type with charge transfer or, in abbreviated form, "CCD" ("Charges Coupled Device") is now produced on the same semiconductive substrate. In these charge transfer registers, each stage can collect charges produced by the light and store these charges. These charges are then transferred from one stage to another to a reading register itself of the CCD type. Thus, each MOS type capacitance can constitute a photosensitive element which is known under the name of photomos, the name under which it is called in the description.
In the image sensors or imagers having large-sized pixels, these pixels consist of photodiodes. These image sensors are generally formed by N lines comprising M photosensitive points each (with N equal to or greater than 1 and M equal to or greater than 1). A surface imager of this type can be obtained by juxtaposing several lines or strips of photodiodes. A CCD type shift register, constituting a reading register of which each stage corresponds to a photodiode or pixel, is associated with each line.
For reading, the charges of each pixel are transferred in the corresponding stage of the reading register (simultaneously for all the lines), then the register is dumped to an output circuit making it possible in particular to convert the charges into voltage; there are therefore, in this case, as many outputs as there are lines.
According to another known structure, the imager is in a matrix form. In this case, on the one hand, each photodiode is connected to a conductor in a column by a switching element often consisting of a MOS type transistor; on the other hand, each column is connected to a stage of a CCD type shift register. The reading is made according to a line by line addressing: in the "conducting" state, all of the switching elements of the same line are controlled so that in each column conductor, the charges accumulated by a photodiode belonging to the addressed line flow. These charges are stored in the corresponding stage of the register, then the latter having been dumped, the addressing of the following line is performed.
In the application to large-sized pixels, the photomos are removed because they present in particular as a drawback the requirement of a relatively long period to dump in a correct way the accumulated or stored charges (the step of the photomos should be compatible with the transfer of the charges).
With the photodiodes, this defect is much less pronounced, but the photodiodes have other serious drawbacks linked to the strong capacitance that they exhibit. This strong capacitance of the photodiodes in particular causes a very significant reading noise as well as a very detrimental hangover or remanence.